We study energetics, electronic and magnetic structures, and magnetic anisotropy barriers of a monolayer of single-molecule magnets (SMMs), ${\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}(\mathrm{COO}R\right)}_{16}]{\left({\text{H}}_2\text{O}\right)}_4$ (abbreviated as ${\mathrm{Mn}}_{12}$, with $R=\mathrm{H}$, ${\mathrm{CH}}_3$, ${\mathrm{C}}_6{\mathrm{H}}_5$, and ${\mathrm{CHCl}}_2$), on a graphene surface using spin-polarized density-functional theory with generalized gradient corrections and the inclusion of van der Waals interactions. We find that ${\mathrm{Mn}}_{12}$ molecules with ligands -H, ${\text{-CH}}_3$, and -${\mathrm{C}}_6{\mathrm{H}}_5$ are physically adsorbed on graphene through weak van der Waals interactions, and a much stronger ionic interaction occurs using a -${\mathrm{CHCl}}_2$ ligand. The strength of bonding is closely related to the charge transfer between the molecule and the graphene sheet and can be manipulated by strain in the graphene; specifically, tension enhances $n$ doping of graphene, and compression encourages $p$ doping. The magnetic anisotropy barrier is computed by including the spin-orbit interaction within density-functional theory. The barriers for the ${\mathrm{Mn}}_{12}$ molecules with ligands -H, -${\mathrm{CH}}_3$ and -${\mathrm{C}}_6{\mathrm{H}}_5$ on graphene surfaces remain unchanged (within $1\mathrm{K}$) from those of isolated molecules because of their weak interaction, and a much larger reduction ($10\mathrm{K}$) is observed when using the ${\text{-CHCl}}_2$ ligand on graphene due to a substantial structural deformation as a consequence of the much stronger interaction. Neither strain in graphene nor charge transfer affects the magnetic anisotropy barrier significantly. Finally, we discuss the effect of strong correlation in the high-spin state of a ${\mathrm{Mn}}_{12}$ SMM and the consequence of SMM-surface adsorption.

We present the results of plane-wave pseudopotential periodic density functional theory (DFT) calculations on the geometries, energetics, and electronic structure of small molecules on Au(111). The chosen molecules—benzene, ammonia and cytosine—are representative of different adsorption regimes and interaction strengths. The chosen substrate is a prototype noble-metal surface that is widely employed as a support for organic materials. We assess the relevance of van der Waals effects in the adsorption process and the accuracy of different first-principle density functionals that have been recently developed to embody such effects. We find that there is no unique functional that is optimal for any system. In particular, our results reveal that functionals designed to reduce the short-term repulsion between the adsorbate and the substrate usually overestimate the adsorption strength and may even predict the wrong adsorption orientation. We show that an accurate description of the substrate does not ensure an accurate evaluation of the adsorption energetics, while the electronic structure is less sensitive to the specific choice. We propose the best choice for DFT calculations of DNA bases on Au(111) and similar systems in which both short-range and long-range interactions exist.

Monolayer graphene grown by chemical vapor deposition and transferred to ${\mathrm{SiO}}_2$ is used to introduce vacancies by ${\mathrm{Ar}}^{+}$ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy is considerably lower than the ion fluence, implying that most of the defects are single vacancies as expected from the low ion energy. The vacancies are characterized by scanning tunneling spectroscopy on graphene and highly oriented pyrolytic graphite (HOPG). A peak close to the Dirac point is found within the local density of states of the vacancies similar to the peak found previously for vacancies on HOPG. The peak persists after air exposure up to 180 min, such that electron spin resonance (ESR) at 9.6 GHz can probe the vacancies exhibiting such a peak. After an ion flux of $10/{\mathrm{nm}}^2$, we find an ESR signal corresponding to a $g$ factor of 2.001–2.003 and a spin density of $1–2\text{spins}/{\mathrm{nm}}^2$. The peak width is as small as 0.17 mT indicating exchange narrowing. Consistently, the temperature-dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of $-10$ K. Thus, the vacancies preferentially couple antiferromagnetically, ruling out a ferromagnetic graphene monolayer at ion induced spin densities of $1–2{\mathrm{nm}}^2$.

The energetically most stable orthorhombic structure of ${\mathrm{OsB}}_2$ and ${\mathrm{IrB}}_2$ is dynamically stable for ${\mathrm{OsB}}_2$ but unstable for ${\mathrm{IrB}}_2$. Both diborides have substantially lower shear strength in their easy slip systems than their metal counterparts. This is attributed to an easy sliding facilitated by out-of-plane weakening of metallic Os-Os bonds in ${\mathrm{OsB}}_2$ and by an in-plane bond splitting instability in ${\mathrm{IrB}}_2$. A much higher shear resistance of Os-B and B-B bonds than Os-Os ones is found, suggesting that the strengthened Os-B and B-B bonds are responsible for hardness enhancement in ${\mathrm{OsB}}_2$. In contrast, an in-plane electronic instability in ${\mathrm{IrB}}_2$ limits its strength. The electronic structure of deformed diborides suggests that the electronic instabilities of $5d$ orbitals are their origin of different bond deformation paths. Neither ${\mathrm{IrB}}_2$ nor ${\mathrm{OsB}}_2$ can be intrinsically superhard.

We report laser-based direct measurements of the pyroelectric and electrocaloric effects of ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ epitaxial layers as a function of temperature and electric field. By evaluating the differences between the total pyroelectric and electrocaloric coefficients, we determine the secondary contribution to the electrocaloric effect that is created by a combination of piezoelectric and elastocaloric effects. The secondary contribution to the electrocaloric coefficient has the opposite sign as the primary effect and therefore reduces the overall entropy change of ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ in an electric field. The absolute magnitude of the secondary effect is comparable to the primary effect at elevated temperatures of ≈200 °C.

We generalize the typical medium dynamical cluster approximation and the local Blackman, Esterling, and Berk method for systems with off-diagonal disorder. Using our extended formalism we perform a systematic study of the effects of nonlocal disorder-induced correlations and off-diagonal disorder on the density of states and the mobility edge of the Anderson localized states. We apply our method to the three-dimensional Anderson model with configuration-dependent hopping and find fast convergence with modest cluster sizes. Our results are in good agreement with the data obtained using exact diagonalization and the transfer-matrix and kernel polynomial methods.

Herein, we study spin-wave dispersion and dissipation in a ferromagnetic insulator–normal metal–ferromagnetic insulator system. Long-range dynamic coupling because of spin pumping and spin transfer lead to collective magnetic excitations in the two thin-film ferromagnets. In addition, the dynamic dipolar field contributes to the interlayer coupling. By solving the Landau-Lifshitz-Gilbert-Slonczewski equation for macrospin excitations and the exchange-dipole volume as well as surface spin waves, we compute the effect of the dynamic coupling on the resonance frequencies and linewidths of the various modes. The long-wavelength modes may couple acoustically or optically. In the absence of spin-memory loss in the normal metal, the spin-pumping-induced Gilbert damping enhancement of the acoustic mode vanishes, whereas the optical mode acquires a significant Gilbert damping enhancement, comparable to that of a system attached to a perfect spin sink. The dynamic coupling is reduced for short-wavelength spin waves, and there is no synchronization. For intermediate wavelengths, the coupling can be increased by the dipolar field such that the modes in the two ferromagnetic insulators can couple despite possible small frequency asymmetries. The surface waves induced by an easy-axis surface anisotropy exhibit much greater Gilbert damping enhancement. These modes also may acoustically or optically couple, but they are unaffected by thickness asymmetries.

We present ${}^{75}\mathrm{As}$ nuclear magnetic resonance measurements on single crystals of $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$, $\mathrm{Ba}{\mathrm{Fe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$, and $\mathrm{Ba}{\mathrm{Fe}}_{1.82}{\mathrm{Cu}}_{0.18}{\mathrm{As}}_2$. While only Co doping induces bulk superconductivity on a broad doping range, the spin fluctuations probed by the nuclear spin-lattice relaxation rate ${\left(T_{1}T\right)}^{-1}$  are identical for both dopings down to $T_c$. Below this temperature, ${\left(T_{1}T\right)}^{-1}$ of the Cu-doped sample continues to rise, proving that (a) there is a quantum critical point below the superconducting dome, and (b) adding electrons does not affect the spin fluctuations. Consequently, we analyze the Knight shift data in terms of a two-component scenario, with one hyperfine coupling to an itinerant degree of freedom and the other to Fe moments.

We study one-dimensional disordered fermions that either undergo metal-insulator transitions or topological phase transitions to become trivial Anderson insulators. We focus on using entanglement to elucidate how the spatial, momentum, and internal degrees of freedom of fermions are affected by the presence of disorder in such cases. We develop entanglement tools that reveal the existence of metallic states in the presence of disorder and further show clear signatures of the corresponding localization transition even in the presence of interactions. In systems where the internal degrees of freedom are coupled with the motion of the electrons, topological phases develop. We subject a topological insulator model to different types of disorder and discuss how the topological aspects of the system can be captured through entanglement, even at strong disorder.

We use quantum Monte Carlo simulations to study a disordered $S=1/2$ Heisenberg quantum spin model with three different nearest-neighbor interactions, $J_1\le J_2\le J_3$, on the square lattice. We consider the regime in which $J_1$ represents weak bonds, and $J_2$ and $J_3$ correspond to two kinds of stronger bonds (dimers) which are randomly distributed on columns forming coupled two-leg ladders. When increasing the average intradimer coupling $(J_2+J_3)/2$, the system undergoes a Néel to quantum glass transition of the ground state and later a second transition into a quantum paramagnet. The quantum glass phase is of the gapless Mott glass type (i.e., in boson language it is incompressible at temperature $T=0$), and we find that the temperature dependence of the uniform magnetic susceptibility follows the stretched exponential form $\chi \sim exp(-b/T^{\alpha })$, with $0<\alpha <1$. At the Néel-glass transition, we observe the standard O(3) critical exponents, which implies that the Harris criterion for the relevance of the disorder is violated in this system.

Temperature-dependent interplane resistivity ${\rho }_c\left(T\right)$ was measured in an isovalent substituted iron-arsenide compound $\text{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x\right){}_2{\mathrm{As}}_2$ over a substitution range from parent compound to slightly below optimal doping $x=0.29$. The feature of interest in the ${\rho }_c\left(T\right)$, a broad resistivity crossover maximum found in the parent compound at $T_{\text{max}}\approx 200$ K, shifts to higher temperatures with Ru substitution, $\sim 340$ K for $x=0.161$ and goes out of the 400 K range for $x=0.29$. Nearly $T$-linear dependence of interplane resistivity is found at the highest substitution level $x=0.29$. This temperature-dependent ${\rho }_c$ and its evolution with substitution bear close similarity to another type of isovalent substituted system ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right){}_2$. Similarly to the isovalent substitutions, the measurements of interplane resistivity in the parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ compound under pressures up to 20 kbar also revealed a rapid rise in $T_{\text{max}}$.

To gain further insight into the properties of interacting topological insulators, we study a one-dimensional model of topological Kondo insulators which can be regarded as the strongly interacting limit of the Tamm-Shockley model. Treating the model in a large-$N$ expansion, we find a number of competing ground-state solutions, including topological insulating and valence bond ground states. One of the effects to emerge in our treatment is a reconstruction of the Kondo screening process near the boundary of the material (“Kondo band-bending”). Near the boundary for localization into a valence bond state, we find that the conduction character of the edge state grows substantially, leading to states that extend deeply into the bulk. We speculate that such states are the one-dimensional analog of the light $f$-electron surface states which appear to develop in the putative topological Kondo insulator, ${\mathrm{SmB}}_6$.

The $GW$ approximation to the electron self-energy has become a standard method for ab initio calculation of excited-state properties of condensed-matter systems. In many calculations, the $GW$ self-energy operator, $\Sigma$, is taken to be diagonal in the density functional theory (DFT) Kohn-Sham basis within the $G_0{W}_0$ scheme. However, there are known situations in which this diagonal $G_0{W}_0$ approximation starting from DFT is inadequate. We present two schemes to resolve such problems. The first, which we called $\mathrm{sc}-\mathrm{COHSEX}+GW$, involves construction of an improved mean field using the static limit of $GW$, known as COHSEX (Coulomb hole and screened exchange), which is significantly simpler to treat than $GW$. In this scheme, frequency-dependent self energy $\Sigma \left(\omega \right)$, is constructed and taken to be diagonal in the COHSEX orbitals after the system is solved self-consistently within this formalism. The second method is called off diagonal-COHSEX $GW$ ($\mathrm{od}-\mathrm{COHSEX}+GW$). In this method, one does not self-consistently change the mean-field starting point but diagonalizes the COHSEX Hamiltonian within the Kohn-Sham basis to obtain quasiparticle wave functions and uses the resulting orbitals to construct the $GW$ $\Sigma$ in the diagonal form. We apply both methods to a molecular system, silane, and to two bulk systems, Si and Ge under pressure. For silane, both methods give good quasiparticle wave functions and energies. Both methods give good band gaps for bulk silicon and maintain good agreement with experiment. Further, the $\mathrm{sc}-\mathrm{COHSEX}+GW$ method solves the qualitatively incorrect DFT mean-field starting point (having a band overlap) in bulk Ge under pressure.

In this work, band gaps of the delafossite Cu-based transparent conducting oxides $\mathrm{Cu}M{\mathrm{O}}_2$ $\left(M=\mathrm{B},\mathrm{Al},\mathrm{Ga},\mathrm{In}\right)$ are calculated by density functional theory (DFT) implemented with many-body perturbation theory (MBPT) based on quasiparticle self-consistent GW approximation (QPscGW) and with Tran-Blaha's modified Becke-Johnson functional (DFT-TB09). Their band gaps are explicitly improved from DFT within local density approximation (LDA). Their optical absorption spectra are also calculated by solving Bethe-Salpeter equation (BSE) that includes the electron-hole correlation effect; they show strong excitonic peaks.

Due to its abundance, silicon is the preferred solar-cell material despite the fact that current silicon materials have indirect band gaps. Although the band gap properties of silicon have been studied intensively, until now, no direct band gap silicon-based material has been found or suggested. We report here the discovery of direct band gap silicon crystals. By using conformational space annealing, we optimize various crystal structures containing multiple (10 to 20) silicon atoms per unit cell so that their electronic structures become direct band gap. Through first-principles calculations, we identify many direct and quasidirect band gap crystal structures, which exhibit excellent photovoltaic efficiency.

We investigate the nonlinear regime of charge and energy transport through Coulomb-blockaded quantum dots. We discuss crossed effects that arise when electrons move in response to thermal gradients (Seebeck effect) or energy flows in reaction to voltage differences (Peltier effect). We find that the differential thermoelectric conductance shows a characteristic Coulomb butterfly structure due to charging effects. Importantly, we show that experimentally observed thermovoltage zeros are caused by the activation of Coulomb resonances at large thermal shifts. Furthermore, the power dissipation asymmetry between the two attached electrodes can be manipulated with the applied voltage, which has implications for the efficient design of nanoscale coolers.